The present invention relates to a process for chlorinating ethylene polymers. By the term ethylene polymers`, the present application refers to both homopolymers of ethylene and copolymers containing at least 50 mol percent of ethylene units, and at least one other ethylinically unsaturated comonomer of 3 - 8 carbon atoms, such as, for instance, propylene, butylene, pentene-1, hexene-1, 4-methylpentene-1, heptene-1and octene-1.
The halogenation of polyolefins, and especially the chlorination of polyethylene, has been known for a considerable period of time, especially for low-density polyethylenes. Note, for instance, U.S. Pat. No. 2,183,556, indicating that this process was known in 1939.
Chlorinated polyethylene has been commercially available since the eary '40s. However, the initial product was expensive and involved difficult methods of preparation, with the resulting polymers exhibiting physical properties which were not as high as desired.
Shortly after the low-pressure polyethylene processes were developed, there was a revival of interest in chlorinated polyethylene, especially the chlorinated polyethylene prepared from so-called high-density polyethylene. Since then, development has continued in the field of chlorination methods for polyethylene, both of the high density and low density types. The methods previously utilized can be catagorized as of three types, the chlorination of polyethylene in solution, in suspension or in bulk.
The chlorination of polyethylene by the solution method is disclosed in, for instance, U.S. Pat. Nos. 2,398,803, 2,748,105, 2,920,064 and 3,060,164. U.S. Pat. No. 2,592,763 discloses the chlorination of polyethylene in an aqueous suspension. The chlorination of polyethylene in a fluidized bed is disclosed in U.S. Pat. No. 2,890,283, and the chlorination of free-flowing polyethylene is disclosed in British Pat. No. 834,905. These two latter processes involve the absence of a liquid dispersing agent or solvent.
The chlorinated polyethylenes which are obtained by the various prior art chlorination methods differ in properties, even if the same starting polyethylene is utilized, and even if the chlorination is effected to the same polymer chlorine content. This difference in properties appears to depend upon whether or not statistical distribution of the chlorine occurs in the chlorinated polyethylene, as explained in French Pat. No. 1,316,044 and British Pat. No. 843,209.
Each of the three prior art chlorination mentioned above, that is, suspension chlorination, solution chlorination and bulk chlorination, involves a number of disadvantages. For instance, solution chlorination is expensive. The solubility of polyethylene in the organic solvents which are inert to chlorine is quite low, so that relatively large quantities of solvent are required. As a rule, expensive halogenated hydrocarbons are used as the chlorination solvent. The chlorinated polyethylene must be freed from the solvent, and freeing the polymer from the last traces of solvent is difficult. The separation is generally cumbersome and expensive. An advantage of the solution chlorination process, however, is that the chlorination reaction proceeds statistically.
At temperatures below the crystalline melting point of the polymer, the chlorination of polyethylene in suspension or in bulk proceeds on a non-statistical basis. A type of block polymer, having chlorinated and unchlorinated segments in the molecule, is obtained unless chlorination is continued until high chlorine contents are achieved. Block copolymers of this type are hard and brittle, whereas in general it is desired to obtain soft and flexible chloropolyethylene, generally having a crystallinity of less than 2% -- that is, the chloropolyethylene should be amorphous or substantially amorphous. In some applications, however, chloropolyethylenes having a higher crystallinity, such as a crystallinity of less than 10%, preferably between 5 and 10%, are desired. A two-stage process for obtaining a more statistical chlorination is described in French Pat. No. 1,216,044. In the process of that patent, the second stage is conducted at a rather high temperature, above the crystalline melting point of the polyethylene (the crystalline melting point, as used herein, is the temperature at which the crystallinity disappears. This temperature differs from the softening temperature, and is generally lower by 15.degree. - 25.degree.C. For instance, the crystalline melting point of high-density polyethylenes is generally between 110.degree. and 120.degree.C, whereas the softening temperature is generally between 130.degree. and 140.degree.C. In the case of low density polyethylenes, these values are generally within the range of 90.degree. - 100.degree.C and 110.degree. - 115.degree.C, respectively). A disadvantage of suspension chlorination at elevated temperatures is that at these high temperatures, even when the reaction is conducted under pressure, the chlorine dissolves in the dispersing agent (which is generally water) only to a very small degree. As a result, the chlorination proceeds quite slowly and very long chlorination times are required.
In British Pat. No. 843,209, the difference between chlorinated polyolefins having a statistical and a non-statistical chlorine distribution are discussed. An intermediate type of chlorine distribution is sometimes obtained on the so-called hybridical polymers. These polymers, in contrast to the process of French Pat. No. 1,316,044, are prepared, as taught by British Pat. No. 843,209, by a chlorination process which is partially effected in suspension and partially effected in solution. This two-stage process, however, is very cumbersome, as the polyethylene chlorinated in suspension must be separated from the suspension and subsequently dissolved, before the second stage can be conducted.
Bulk chlorination, for instance, by the process described in the above-mentioned U.S. Pat. No. 2,890,213 and British Pat. No. 834,905, can be conducted at rather high temperatures, at which the chlorination proceeds rapidly. However, a disadvantage of this chlorination process resides in the fact that during the initial phase of chlorination the reaction proceeds too fast, even if the initial phase is conducted at relatively low temperatures. Halogenation reactions in general and chlorination reactions specifically are exothermic reactions. In the chlorination of finely divided polyethylene, without a solvent or dispersing agent, heat is locally developed, causing the reaction speed to increase in the hot spots so developed, with increased heat development, and generally it is impossible for the heat to be discharged at a sufficient rate. This causes melting, and consequentially lump formation of the chlorinated polymer, as well as decomposition or even combustion to occur. If combustion occurs, generally a highly undesirable discoloration will result. Because of the fact that melting occurs locally, lumps may be formed whose subsequent chlorination proceeds in an extremely poor and irregular manner.
With bulk chlorination it is almost impossible to avoid polymer discoloration, unless the chlorination is conducted at such low temperatures that the reaction speed is quite slow, even in the presence of an initiator. In addition, at temperatures below the crystalline melting point of the polymer, no statistical chloropolyethylene will be obtained. Often the discoloration of the polymer is so serious that the resulting chlorinated polyethylene is of a brown color.
In order to improve the properties of the resulting chloropolyethylene, to increase the degree of chlorination, and for other purposes, a numer of two-stage processes have been proposed, as mentioned in the abovecited French Pat. No. 1,316,044 and British Pat. No. 843,209. In addition, U.S. Pat. Nos. 2,398,803 and 2,920,064, German Auslegeschrift No. 1,420,451, British Pat. Nos. 1,073,504 and 1,036,360 describe the chlorination of polyolefins in two stages.
None of the prior art processes for chlorinating polyethylene to chloropolyethylene have been quite satisfactory. In particular, these prior art processes for the preparation of chloropolyethylene having a statistical chlorine distribution, or the preparation of hybridical chloropolyethylene, leave much to be desired.